Transmissible gastroenteritis (TGE) of swine is caused by a coronavirus (TGEV). The disease occures wherever swine are raised, and it is highly fatal for baby pigs during the first few weeks after farrowing. However, if the sow has had a recent TGE infection, protective antibodies are transferred to the offspring in the colostrum and milk which protect the baby pigs against TGE. Bohl et al, Infect. & Immun., 6, 289-230 (1972). TGE vaccines have been developed which are capable of producing high serum virus neutralizing titers to TGEV. Such vaccines are prepared from attenuated, non-virulent TGEV, strains of which may be prepared by continuous cell culture of virulent TGEV. See, for example, U.S. Pat. Nos. 3,479,430 and 3,585,108. However, such vaccines are not very effective in producing lactogenic immunity in the sow, whether the attenuated TGEV is given live or inactivated. Tamoglia, J.A.V.M.A., 160, 554-558 (1972).
Studies have shown that IgA antibodies are produced in milk by TGE infection or by administration of live virulent TGEV, while the administration of attenuated vaccines produces mainly IgG antibodies. Bohl et al, Infect. & Immun., 11, 23-32 (1975). Bohl et al suggest that effective passive immunization of baby pigs may depend on continual neutralization of virus within their intestinal tracts by the IgA antibodies produced in milk after exposure of the sow to virulent TGE. However, vaccines containing virulent TGEV would be hazardous to use, since the virus is readily transferred among swine in a herd.